The present invention relates to a laminate base material made of a combined non-woven fabric mainly comprising aromatic polyamide fibers, and a method of producing it. The present invention also relates to a prepreg and a laminate made of the laminate base material.
As electronic equipment is becoming compact and the density thereof is becoming higher recently, components to be mounted on a printed board are being changed from those of insertion mounting type to surface mounting type and, accordingly, the mounting on the printed board is mainly conducted by the surface mounting method. In this method, the connection reliability between the components such as chips to be mounted on the surface and the printed board is very important. Namely, the coefficients of thermal expansion of both of them should be as close to each other as possible. The coefficient of thermal expansion of the chips of recent thin surface mounting type is 5.times.10.sup.-6 /.degree. C., and that of laminates prepared by impregnating a non-woven glass cloth with an epoxy resin is about three times as high as this.
The dielectric constant is also to be considered at some points. Usually, the dielectric constant of ordinary FR-4 is about 4.7 to 5.1. Since such a relatively high dielectric constant delays the propagation of the electric pulse of the adjacent signal circuit, excessive signal delay is caused. When the frequency is high, the delay of the signal propagation in the printed board is a serious problem. In such a case, a laminate base material of a low dielectric constant is necessitated. The term "FR-4" indicates a copper-clad laminate for printed boards, which is produced by impregnating sheets of a glass cloth base with an epoxy resin and laminating them (NEMA number of JIS).
Under these circumstances, intensive investigation has been made for the purpose of using laminates, produced by using a non-woven fabric of aromatic polyamide fibers having a negative coefficient of thermal expansion and a low dielectric constant, as the laminates (including metal foil-clad laminates) to be used as the base materials for printed boards. The non-woven fabrics include (1) those prepared by combining a p-phenylene terephthalamide fiber flock with a m-phenylene isophthalamide fibrid to form a sheet and then compressing the sheet under heating [for example, Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J. P. KOKOKU") No. Hei 5-65640] and (2) those obtained by combining stretched fibers of p-phenylene diphenyl ether terephthalamide with non-stretched fibers of m-phenylene isophthalamide to form a sheet, then bonding the fibers with one another by means of a resin binder and compressing the sheet under heating. The resultant laminate base material is impregnated with a thermosetting resin and compression-molded under heating to obtain the laminate. Usually, a metal foil which is processed into a printed wiring is applied to the laminate in the heat compression molding step to obtain an integral metal foil-clad laminate.
The base material (1) made of the film of the combined p-phenylene terephthalamide fiber flock and the m-phenylene isophthalamide fibrid is highly hygroscopic and, therefore, when it is used for forming a printed board, tan .delta. and the dielectric constant are increased by the moisture to cause electric problems. Further, since no resin binder is contained therein, a high pressure is necessitated for the compression with a hot roll so as to obtain an intended density, thereby increasing the fluctuation of the density.
On the other hand, although the hygroscopicity of the base material (2) made of the combined sheet comprising stretched fibers of p-phenylene diphenyl ether terephthalamide and non-stretched fibers of m-phenylene isophthalamide is more improved than that of (1), there is a room for a further improvement. In addition, the bonding power of the non-stretched fibers of m-phenylene isophthalamide fibrid by means of a hot roll in the heating/compression process is lower than that of the m-phenylene isophthalamide fibrid. It was found that the difference in the bonding power between them causes a serious warping of the printed board after the heating step.
Attempt was made, therefore, to reduce the hygroscopicity of the thermosetting resin used for the impregnation of the laminate base material so as to reduce the hygroscopicity of the printed board prepared therefrom. However, no remarkable effect on the hygroscopicity of the printed board was obtained because the hygroscopicity of the thermosetting resin is essentially low. In addition, since the warp of the printed board (2) is mainly due to the bonding power of the m-phenylene isophthalamide fibrid, this defect cannot be overcome with the thermosetting resin to be used for the impregnation of the laminate base material.